2006MNRAS.368.1971B

We have obtained BVI CCD imaging of Berkeley 17, an anticentre open cluster that competes with NGC 6791 as the oldest known open cluster. Using the synthetic colour-magnitude diagrams (CMDs) technique with three sets of evolutionary tracks, we have determined that its age is 8.5-9.0 Gyr, it distance modulus is (m - M)₀= 12.2, with a reddening of E(B - V) = 0.62-0.60. Differential reddening, if present, is at the 5 per cent level. All these values have been obtained using models with metallicity about half of solar (Z = 0.008 or 0.01 depending on the stellar evolution tracks), which allows us to reproduce the features of the cluster CMD better than other metallicities. Finally, from the analysis of a nearby comparison field, we think to have intercepted a portion of the disrupting Canis Major dwarf galaxy.